This invention relates to a system for the proportional electrohydraulic remote positioning of a plurality of hydraulic control valves.
In the use of hydraulically operated working machines, especially load-handling apparatus, such as vehicle-mounted cranes of the type finding use in mechanized lumbering, it is often desirable to be able to effect the control by means of a portable control unit, that is, from a location that can be chosen according to need in each case. Various embodiments of systems permitting such control are known.
In a known type of system of this kind, the entire hydraulic equipment is disposed on the working machine and connected to a control box through an electric cable. The hydraulic equipment comprises a plurality of proportional control valves in the form of directional control valves for the distribution of hydraulic liquid to the working cylinders, each such control valve be operated by a hydraulic positioner and an electrohydraulic converter connected to the positioner.
The positioner commonly is a double-acting hydraulic cylinder and the converter is an electromagnetically operated servo valve device. This servo valve device is supplied with an electric control signal set at the control box and, depending on the direction or sign of this signal, delivers hydraulic liquid to either of the two compartments of the positioner cylinder until the pressure in that compartment has reached a value proportional to the magnitude of the control signal, namely, until the piston of the positioner cylinder has displaced a spring-centered valve spool of the control valve a distance proportional to the magnitude of the control signal.
The control box includes a control signal transmitter having lever-operated potentiometers which are associated with respective ones of the control valves to be proportionately positioned. Each potentiometer is connected to a pair of solenoids of the converter of the associated control valve such that, depending on the direction of the deflection of the potentiometer lever, one or the other of the two solenoids is supplied with an electric control signal proportional to the magnitude of the lever deflection, which control signal the converter then translates into a proportional hydraulic positioning signal for the positioner.
Since the hydraulic equipment may comprise several control valves -- six control valves are common -- and since each control valve requires two conductors in the electric cable between the working machine and the control box (these conductors have to be capable of passing a current of a few hundred milliamperes), the cable is expensive and, above all, heavy and thus difficult to handle. This disadvantage is particularly apparent when the working machine is used in forests, since the cable then tends to get caught in scrub, low bushes or tree-stumps. Another disadvantage of systems of the known type is that because of the friction of the valve spools, the remote positioning of these spools cannot be made free from hysteresis, unless special steps are taken.